1. Field of the Invention
This invention relates to motor driven pumps.
2. Description of the Prior Art
Many designs of motor driven pumps of the type referred to as canned motor pumps, have heretofore been proposed among which are those shown in U.S. Pat. Nos. to White, 2,713,311; 2,741,990; 2,763,214; 2,796,835; 2,906,208; 2,913,988; 3,053,189; 3,111,090; 3,138,105; 3,220,349; 3,220,350 and 3,280,750 and Litzenberg, U.S. Pat. No. 2,871,791.
A canned motor pump is a hybrid device utilizing the arts and techniques required in the design of an electric motor and the arts and techniques required in a hydraulic device, such as a pump, and wherein the design parameters, and other factors are entirely different and unrelated. An electric motor is an extremely close tolerance device with very precise alignment requirements, particularly those that pertain to the air gap between the rotor and stator elements. In addition, the motor efficiency is critically affected by the distance of the air gap between the two parts. Since, in a canned motor pump, shielding is required of the inside of the stator and the stator and the exterior and ends of the rotor with a corrosion resistant metal to protect these parts from the fluid being handled this makes it necessary to increase the air gap from the normal 0.020 inch clearance to an 0.060 inch clearance.
A unit of this type commonly utilizes sleeve bearings of various materials which when new and not worn have clearances of the order of 0.003 inch to provide for a fluid film for bearing lubrication and bearing cooling.
From the foregoing it will be apparent that alignment and close tolerances are of the utmost importance in order to obtain originally the minimal efficient air gap and to provide for wear on the bearings that results from use.
As in any device the standards must be established for the entire device based upon the demands of the most critical portions of the unit. While the hydraulic components do not require the same precision requirements they must be designed, manufactured, and utilized with the same tolerance factors as the motor. In the past, available designs developed and marketed obtained alignment at high cost with service difficulties, in part interchangeability and restricted service life.
The post design herein described solves the limited service life, part interchangeability and critical alignment problems heretofore encountered. In this post design all parts relate to only one part for the tolerance considerations. The post controls all the other parts essentially in a one to one relation so that there are only two tolerances, one is with respect to the post and the other is with respect to any other part to the post without stacking or compounding of the parts with each other and piling up or pyramiding tolerances in each of the parts as they relate to the others.
The U.S. Pat. No. to Sence, 3,128,712 for canned motor pump is illustrative of the pyramiding of tolerances which are avoided with the present invention.
If in Sence the housing 30 can be taken as a starting point and as having a basic tolerance, part 22 (front is assembled, then part 16 is assembled to part 22 (front) and part 22 (rear) is added, both of these in a relationship to the liner 24 and then to these is added part 35 (bearing housing).
While all these parts stem from the bearing housing they are connected in a series arrangement but in no direct relation to the bearing housing 30 except through their tolerances.
From a practical standpoint fine machine tolerances on parts of this type will at high cost have tolerances of at least 0.002 inch so that if all the parts are exact there is a tolerance build up for the five tolerance locations of at least 0.010 inch which causes a misalignment of 50% of the motor air gap. A misalignment of this nature reduces considerably the efficiency of the motor, causes problems in part interchangeability which affects both life performance and field service repairs and reduces considerably the available bearing wear life. While this discussion relates to the motor section it is similarly applicable on the hydraulic side which further compounds the problem.
With the post design herein described and by the use of a unitary motor housing in which the machining can be carried out in a single operation for close control of the tolerances the objectionable pyramiding of tolerances as referred to above is avoided and with bearing alignment and motor rotor air gap simply but effectively obtained and maintained.
The patent to White, U.S. Pat. No. 2,796,835 shows a motor driven pump which, like Sence, has a pyramiding of tolerances in an undesirable fashion.
White at 53 has a fixed solid mandrel which does not permit the continuous flow of a portion of the liquid being pumped for bearing cooling and lubrication and pressure balancing.
Other prior art patents now known also lack any teaching of the post concept and have pyramided tolerances inherent in their structures.
The basic design parameters of all canned motor pumps involve a wet section which contains the fluids being pumped and the elements for that purpose including the motor rotor shaft assembly, impeller and bearings. The dry section is that of the motor stator which is separated from the wet section by a non-magnetic liner or sleeve between the rotor and the stator.
In the previous designs all parts in both the wet and dry sections were interdependent on each other thereby establishing an excessive tolerance build up due to intermingling of the close tolerance requirements of the electric motor components and the greater tolerances in the hydraulic section.
The motor driven pump of the present invention permits of complete divorce of the wet section from the dry section.
The motor driven pump of the present invention, in addition to the advantages referred to is also capable in a single size of accommodating different motor sizes thus reducing the cost of manufacture.
The constructions heretofore employed, also, were not suited for a range of sizes but individual designs of pumps of small range were common practice. It was not considered feasible to use a single design which with minor changes in components, change of pump inlet and outlet sizes, or change of length of the motor stator and/or of the motor rotor, to effect accommodation for a different input, or capacity, or other pump characteristics.
The canned motor pump or motor driven pump of the present invention has other advantageous features. The inboard bearings because of their location in a cylindrical tubular shaft are always in alignment. Better distribution of work loads both hydraulic and electrical is available over the bearings. The bearings can be readily replaced and can be made of the desired length for the load because ample space is available.
A simplified motor driven pump is provided which overcomes many of the shortcomings of the motor driven pumps heretofore available.